st/xlib Fix XIMage bytes-per-pixel calculation
st/xlib Fix XIMage bytes-per-pixel calculation

Fixes a crash seen while running gnome on a 16-bit screen over vnc.

NOTE: This is a candidate for stable release branches.

Reviewed-by: Brian Paul <3cb4e1df5ec4da2c7c4af7c52cec8cf340a55a10@vmware.com>
Signed-off-by: Richard Sandiford <d14feb13b187709a6a8e3489dc080708170d5b05@linux.vnet.ibm.com>

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